225 lines
3.3 KiB
NASM
225 lines
3.3 KiB
NASM
; Bootsector for FAT32 filesystems
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org 0x0800
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cpu 8086
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jmp short init
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nop
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%include "inc/bpb.asm"
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bpb: equ 0x080B
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; Area for BPB
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times ( 0x5A - ($-$$) ) db 0
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init: xor ax, ax
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mov ss, ax
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mov sp, 0x7C00
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; save potential partition table entry
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push ds
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push si
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mov ds, ax
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mov es, ax
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mov si, 0x7C00
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mov di, 0x0800
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mov cx, 0x0100
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rep movsw
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jmp 0x0:main
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%include "inc/booterr.asm"
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; offsets relative to FS
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readd: ; add offset of cluster data area to DX:AX
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push cx
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mov di, [bp+bpb_rde]
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mov cl, 4
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shr di, cl ; 32 bytes per entry
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add ax, di
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adc dx, 0
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pop cx
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readr: ; add offset to rootdir to DX:AX (FAT12/FAT16 only)
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push cx
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xor ch, ch
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mov cl, byte [bp+bpb_fn]
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fatlp: add ax, [bp+bpb7_lsf]
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adc dx, [bp+bpb7_lsf+2]
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loop fatlp
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pop cx
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readf: ; add offset to FAT table to DX:AX
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add ax, word [bp+bpb_rsc]
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adc dx, 0
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readp: ; read sector DX:AX from partition
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add ax, word [bp+bpb_po]
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adc dx, word [bp+bpb_po+2]
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jc dskerr
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read_: ; read sector DX:AX from disk
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; qword sector number DX:AX
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push cs
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push cs
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push dx
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push ax
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; dword target buffer: BX:0
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push bx
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push cs
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; word sector number
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push cx
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; size & passing
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mov di, 0x10
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push di
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mov si, sp
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mov ah, 0x42
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mov dl, [bp+bpb7_dn]
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stc
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int 0x13
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jc dskerr
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add sp, di
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ret
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; error handling for disk errors
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dskerr: call errcll
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next: ; Advances DX:AX to next FAT entry
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push ax
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push bx
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; shift 2 left for dword-sized FAT entries
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; shift 9 right for sector size
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mov cl, 7
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shftl: clc
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rcr dx, 1
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rcr ax, 1
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loop shftl
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mov bx, 0xA0
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mov cx, 1
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call readf
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pop bx
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; get lower part of cluster number
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pop si
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; multiply with 4
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sal si, 1
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sal si, 1
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; make sure its within sector range
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and si, 0x1FF
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add si, buf
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; load dword from FAT
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lodsw
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mov dx, [si]
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ret
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; reads current cluster into [dest]
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readc: ; load cluster number
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push ax
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push dx
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; subtract the two dummy entries from FAT start
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sub ax, 2
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sbb dx, 0
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; convert cluster number to sector number
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; this is some cheapo multiplication with 2^n
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mov cl, [bp+bpb_sc]
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l02: shr cl, 1
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jz l02e
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clc
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rcl ax, 1
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rcl dx, 1
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jmp l02
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l02e:
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; dx:ax is now sector num in data area
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mov cx, [bp+bpb_sc]
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xor ch, ch
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call readd
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xchg di, cx
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mov cl, 5
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sal di, cl
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add bx, di
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pop dx
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pop ax
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ret
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; load cluster chain DX:AX to [dest]:0
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load: mov bx, 0x07C0
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lloop: call readc
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call next
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cmp word dx, 0x0FFF
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jne lloop
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ret
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; Search for first entry with matching fsattr
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search: mov si, (0x7C00-0x20)
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loop: add si, 0x20
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; check if entry is valid
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mov al, [si]
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; unallocated direntry
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test al, al
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jz fserr
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; deleted files
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cmp al, 0xE2
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je loop
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; check attr
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mov al, [si+11]
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and al, 0x5C
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cmp al, ah
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jne loop
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ret
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loadr: mov ax, 2
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xor dx, dx
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jmp load
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loadf: call search
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jne fserr
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mov ax, [si+0x1A]
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mov dx, [si+0x14]
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jmp load
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main: mov bp, bpb
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mov [bp+bpb7_dn], dl
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; load root directory
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call loadr
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; search for first system directory
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mov ah, 0x14
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call loadf
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; search for first system file
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mov ah, 0x04
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call loadf
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; jump
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xor ax, ax
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xor cx, cx
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xor dx, dx
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xor bx, bx
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xor di, di
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; restore drive number
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mov dl, [bp+bpb7_dn]
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; restore potential partition table
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pop si
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pop ds
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jmp 0:0x7C00
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; error handling for file not found
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fserr: call errcll
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; Padding and signature
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times (0x1FE - ($-$$)) db 0
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dw 0xAA55
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buf:
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